(1) Field of the Invention
The present invention relates to a method for driving an active display, and more particularly relates to a driving method with a reset process.
(2) Description of the Prior Art
An organic light emitted diode (OLED) is an illumination device activated by electric current. The illumination of the OLED is changed with the applied current. Active elements for activating the BLED in present can be classified into low temperature polysilicon thin film transistor (LTPS-TFT) and amorphous silicon thin film transistor (a-Si TFT). The LTPS-TFT is widely used nowadays. Whereas the a-Si TFT with the advantages of fewer lithographic steps and lower temperature in the fabrication process is preferred to be the trend in the future. However, both the LTPS-TFT and the a-SI TFT has a problem that the current passing through the OLED is decreased due to the increasing of threshold voltage. The problem is more significant for the OLED using a-Si TFT as active elements.
As an amorphous-TFT based OLED panel is operated, a high electric current passes through the channel of the a-Si TFT. Therefore, the electrons are easily trapped in the gate dielectric to raise the threshold voltage of the a-Si TFT and decrease. As a result, the electric current is decreased to reduce the illumination of the OLED and badly influence the life of the OLED panel.
In present, for solving the above mentioned problem, a typical method is to apply an electric field from the source/drain electrode of the TFT toward the gate electrode for driving the electrons away from the gate dielectric to recover the original threshold voltage. There two embodiments for the method:
First, as shown in FIG. 1A, the source/drain electrode of each pixel is applied with a positive resetting voltage Vs′ or Vd′ respectively. Thereby, the electric field from the source/drain electrode pointing to the gate electrode is formed to release the electrons trapped in the gate dielectric back to the channel of the TFT.
Second, as shown in FIG. 1B, the gate electrode of each pixel is applied with a negative resetting voltage Vg′. Thereby, the electric field from the source/drain electrode pointing to the gate electrode is formed to release the electrons trapped in the gate dielectric back to the channel of the TFT.
Also referring to FIG. 1C, as the electric field is formed to release the electrons 13. The electrons 13 trapped in the gate dielectric 11 of the TFT 10 have the motion opposite to the direction of the electric field back to the channel 14 so as to recover the amount of free electrons within the channel 14 to prevent the increasing of the threshold voltage.
In the related art, as the gate driving voltage Vg is remained, the source/drain electrode should be applied with a greater positive voltage to increase the drain/source voltage Vd, Vs to the resetting voltage Vs′, Vd′. As the drain voltage Vd and the source voltage Vs are remained, the gate electrode should be applied with a greater negative voltage to decrease the gate driving voltage Vg to the gate resetting voltage Vg′. The greater positive voltage or negative voltage applied to the gate dielectric may reduce the operation efficiency of the OLED.